2.1 Field
The field of the invention is delayed coking. More particularly, the field of the invention is methods and mechanisms for dispensing fluid to delayed coke drums.
2.2 Description of Related Art
In delayed coking, heavy distillation fractions (“resid” or “residuum”) is typically heated rapidly in a fired heater or tubular furnace to create a mixture of hot liquid and vapor which is then fed to a large steel vessel commonly known as a coke drum. The coke drum is maintained under conditions in which coking occurs (e.g., greater than about 400° C. under super-atmospheric pressures). Delayed coke drums are typically cylindrical vessels with a cone at the bottom; that range in diameter anywhere from about 15 to 30 feet. The height of a delayed coke drum is typically two to five times the diameter.
During the delayed coking process, the heated resid undergoes high temperature decomposition to produce more valuable liquid and gaseous products and solid or semi-solid coke residue. The volatile components are removed overhead and pass on to a fractionator. The solid or semi-solid coke left behind accumulates in the drum. When the coke reaches a certain level, a switch valve is moved to redirect the resid to an empty “sister” drum. The hydrocarbon vapors in the full drum, now off line, are then purged with steam and the drum is quenched with steam and water to lower the temperature to less than about 100° C.—after which the water is drained. When the cooling and draining steps are complete, the top and bottom heads of the drum are opened and the coke is removed by drilling and/or cutting. For example, high velocity water jets may be lowered in through the top of the drum.
Typically, each end of a delayed coking drum is capped with a bolted on steel plate called a “head.” The process of removing the top and bottom heads of a coke drum is called “unheading” or “deheading.” There are several conventional methods for opening the heads of a coke drum. One method is to completely remove the bottom head from the vessel and, optionally, carry it away on a cart. Another method is to swing the bottom head out of the way, as on a hinge or pivot, while the head remains coupled to the vessel. (See e.g. U.S. Pat. No. 6,264,829.) Manually removing the heads, especially the bottom heads, is dangerous work and has resulted in serious injuries and fatalities. Operators face significant risk of injury from exposure to steam, hot water, coke fallout, fire, etc. To help alleviate this risk, the industry has developed semi-automatic or fully automatic systems for the bottom unheading.
From the late 1930s through the 1950s, heated resid was predominately fed to delayed coke drums through a single horizontal side-inlet in a side wall near the bottom of the drum. There are several problems with this design, as illustrated in N. A. Weil and F. S. Rapasky, “Experience with Vessels of Delayed Coking Units,” Proceedings of the American Petroleum Institute, Section III Refining, pp. 214-232 (1958). Basically, when the heated resid enters the coke drum, it shoots across the drum against the wall opposite the inlet. Thus, the wall opposite the inlet is subjected to higher heat than the remainder of the drum. The thermal shock caused by this non-uniform heat distribution expresses itself in a number of ways, including: recurrent plastic deformation of the coke drum bottom and eventual ovalization; leaks in nearby gasketed joints; metal fatigue; and cracks in the drum.
From the late 1950s to the early 2000s, with some exceptions, the side inlet feed design was replaced with a single vertical bottom-inlet design. Relative to the single side-inlet design, this configuration reduced the non-uniform temperature distribution and concomitant leak problems. Typically, the bottom feed inlet is through the center of the bottom head and the feed line is disconnected before the bottom head is removed.
Over a many years, actuated severe service valves have been suggested in the industry by a number of vendors as a safer and more time efficient alternative to the use of bottom heads on delayed coking drums. Since about 2001, suitable valves for this purpose have been disclosed by, among others, Zimmermann and Jansen GmbH, Curtiss-Wright Flow Corporation and Velan Inc. (See e.g., Zimmermann and Jansen GmbH U.S. Pat. Nos. 5,116,022 and 5,927,684, Curtiss-Wright Flow Control Corporation U.S. Pat. Nos. 6,656,5714, 6,666,0131, 6,843,889, 6,964,727, 6,989,031 and 7,033,460 and Velan Inc. U.S. Patent Application No. 2005/0269197). However, if one replaces a coke drum bottom head with a severe service valve, the concurrent use of a vertical bottom feed-inlet becomes much more problematic and, in some cases, impossible. To be repetitively and continuously operable through numerous coking/decoking cycles without removal, this type of valve closure requires a lateral feed system that is located above the valve apparatus. As a result, the industry is moving back to the use of a single horizontal side-inlet feed nozzle despite the associated thermal stress problems. This is illustrated, for example, in U.S. Patent Application No. 2004/0251121.
Two published patent applications, namely, US Patent Application No. 2004/0200715 and US Patent Application No. 2004/0251121, have attempted to address the stress induced leakage problems encountered in coke drums when a valve is used as a bottom head in combination with a single side feed inlet. These proposed solutions treat the symptom rather than the disease by focusing on valve insulation and seal design to increase thermal stress resistance rather than the uneven feed distribution that causes the thermal stress.
U.S. Pat. No. 7,115,190 (“the '190 patent”) describes “a tangential injection system for use within a delayed coking system . . . . The tangential injection system comprises a spool, [and] a tangential dispenser, . . . wherein the tangential dispenser comprises a delivery main surrounding the perimeter of the spool that functions to deliver a residual byproduct . . . to a plurality of feed lines positioned . . . at distances around the delivery main for the purpose of providing tangential dispensing of the residual byproduct into the vessel, thus effectuating even thermal distribution throughout the vessel.” The complexity of the tangential injection system described in the '190 patent is self-evident from the patent itself. “[F]riction forces tend to create a reduction in the velocity of the residual material as it travels through [the] curved pipe section . . . . As such, these forces are taken into consideration when designing the size and location of each of [the] feed lines . . . , their respective angles of entry, as well as the respective cross-sectional areas of each feed line and delivery main.” See the '190 patent col. 7, lines 46-55. “The relative sizes of the plurality of feed lines may vary so that the volume and/or velocity passing through the lines in [sic] somewhat equalized . . . . ” See the '910 patent col. 8 lines 6-8.
The '190 patent also “illustrates . . . [a] prior art dispenser . . . namely a system comprising two opposing, co-axial inlet feeds coupled to a vessel in the form of a coke drum.” See the '190 patent col. 4 lines 55-59. The '190 patent then asserts that “[a]lthough the addition of another dispenser or inlet feed helps alleviate some of the problems discussed above . . . namely the lack of uniform heat distribution, the remedial effect or benefit of two opposing inlet feeds on these problems is only minimal. A significant amount of uneven heat distribution and thermal variance still exists within or throughout [the] vessel because of the inability of the inlet feeds . . . to dispense byproduct in a controlled and predictable manner.” [Italic emphasis added]. “For example, byproduct from each feed inlet . . . is dispensed into the vessel. If the pressure within each inlet feed are similar, the byproduct from each feed inlet will meet somewhere in the middle and cause the byproduct to be randomly displaced within [the] vessel . . . . On the other hand, in the even [sic; event] that a pressure differential exists between inlet feeds . . . then the byproduct will be even more randomly dispensed and the problems of thermal variance increased.” See the '190 patent col. 3 lines 6-22.
There remains a need to provide easier and more effective solutions to the high thermal stress problems caused by the lateral side introduction of heated resid to coke drums in view of the industry's desire to replace coke drum bottom heads with valves.